Right, I just had the following chat online with O2 CS. Pretty much as I thought, they WILL provide nano-SIMs if required:
Welcome to O2. Someone will be with you soon.
We're really busy right now. Thanks for waiting.
You're through to Akhil.
Akhil: Hi I'm O2 : Akhil. How can I help?
XXXXXXX: Hi. As an existing O2 customer, I am hoping to buy a new iPhone 5 direct from apple when they are released.
Akhil: Okay
XXXXXXX: I have an iPhone 4S at present, which uses a micro-sim. The new handset is rumoured to be using a nano-sim. If this is the case, will O2 be able to provide me with a new nano-sim?
Akhil: Sure
Akhil: You'll get the compatible SIM free of cost from O2
XXXXXXX: When I bought my 4S and required a micro-sim, I just had to come in-store with proof of purchase. Will it be the same deal again with the nano-sim?
Akhil: No
Akhil: You need to just contact us
Akhil: We'll send you SIM along with phone
XXXXXXX: No, you misunderstand. I will be buying the phone at the apple store direct.
Akhil: Okay
Akhil: I'm sorry
Akhil: Then you need to contact us for SIM
Akhil: We'll send the SIM to you at your address
XXXXXXX: But you are still able to do this ok for me?
Akhil: I'm sorry that SIM is not available now
XXXXXXX: That's ok, I understand. I just want to clarify that O2 will indeed supply me with a nano-SIM if required, even if I am buying my new handset direct from Apple.
Akhil: Sure
Akhil: You'll get this SIM from O2 without any cost
XXXXXXX: That's great. Thanks very much for your help.
XXXXXXX: Have a nice day.
Akhil: You're welcome XXXXXXX
Akhil: Thank you
Akhil: You too